Electrical discharge arcing may occur in a power transmission system as a consequence of an overvoltage condition arising from a switching event or a lightning strike, as a consequence of breakdown or bridging of insulation between conductors at different potentials and as a consequence of thermal destruction of circuit devices. In some cases arcing is accommodated (for example in use of arcing horns) for the purpose of protecting equipment against overvoltage induced fault currents until such time as the current supply is terminated by operation of circuit breakers, whilst in other cases unpredictable or, at least, unwanted flashover-type arcing occurs in or adjacent to electrical equipment. However, in all cases in which arcing is sustained it may provide a starting point for fires and/or may cause major damage to plant and equipment.